1. Field of the Invention
This invention relates to a wet type developing device, and, more particularly, it is concerned with a wet type developing device of a construction, in which a liquid developer is conveyed on a porous, resilient member having a liquid absorbing property and a liquid exuding property under pressure so as to feed the liquid developer to an image carrying body.
2. Description of the Prior Art
In laid-open Japanese patent application No. 52-40336, for example, there is disclosed a device of a construction, in which developing liquid is conveyed on a developing roller comprising a roller made of an electrically conductive, resilient foamed material and a resin net covering the surface of the resilient foamed roller. In this developing device, the developing roller is immersed at its lower part into a developing liquid accommodated in a liquid vessel, while its upper part is press-contacted to an image carrying body. At this press-contacted portion, the roller is subjected to elastic-deformation to ooze out the developing liquid impregnated therein and to feed the same to the image carrying body. The developing roller also has a refreshing roller press-contacted thereto in the liquid vessel. The refreshing roller has the functions of elastically deforming the developing roller to exude the developing liquid therefrom after use, and of absorbing fresh developing liquid thereinto at the time of its reinstatement into the original shape from the elastically deformed state.
In this developing device, it is not impossible to apply a bias voltage to the developing roller from the refreshing roller which is made an electrode. In view, however, of the fact that an insulative net is interposed between the electrode roller and the electrically conductive foamed body of the developing roller, the contact between the electrode roller and the electrically conductive foamed body is not stable, hence a developing potential to be formed on the developing roller is also not stable. A more favorable developed image can be obtained when the core of the electrically conductive foamed body of the developing roller is made of a metal, and the developing bias voltage is applied to the electrically conductive foamed body through this metal core. Even in this instance, too, it may happen that the electrically conductive foamed body of the developing roller contacts the refreshing roller at the press-contacted portion of both rollers through the insulative net, whereby the developing bias voltage inevitably leaks from the electrically conductive foamed body to the refreshing roller. By this leakage of the developing bias voltage, there occurs irregularity in the image density or irregular ground fogging over the developed image. Further, when the developing bias voltage is applied to the electrically conductive foamed body of the developing roller, if an intense electric field is generated across the developing roller and the refreshing roller, the following inconvenience would arise. That is, assume that the toner in the developing liquid consisting of the toner dispersed and suspended in the insulative carrier liquid is charged to the negative polarity. In case the refreshing roller functions equally to the positive polarity with respect to the developing roller, the toner moves from the developing roller to the side of the refreshing roller to lower the toner density in the liquid impregnated in the developing roller with the consequence that density of the developed image lowers. Further, the toner in muddy form accumulates on the surface of the refreshing roller. On account of this, a multitude of fine openings on the surface of the developing roller are clogged with the muddy toner, whereby the developing roller reduces its liquid absorbing and exuding properties, and the density of the developed image becomes much lower or non-uniform. Furthermore, due to muddling of the toner, toner consumption rapidly increases. Conversely, when the refreshing roller functions equally to the negative polarity with respect to the developing roller, the developing capability becomes unstable. In other words, as the result of excessive quantity of the toner moving toward the developing roller, the excess toner accumulates on the surface or in the interior of the developing roller to cause unreasonable increase in density of the developed image, or clogging of the fine openings in the developing roller to deteriorate its liquid absorbing and exuding properties. As the result of this, the image density conversely lowers or becomes nonuniform.